The Shiller P/E ratio, a measure of long-term market valuation, has only crossed 40 three times: 1929, 1999, and today. The first two instances preceded major market crashes (The Great Depression, Dot-com Bust) and were followed by a decade or more of flat or negative real returns for investors.

During periods of intense market euphoria, investors with experience of past downturns are at a disadvantage. Their knowledge of how bubbles burst makes them cautious, causing them to underperform those who have only seen markets rebound, reinforcing a dangerous cycle of overconfidence.

Despite his reputation, Marks made just five significant macro calls in his career. These were not based on economic forecasts but on 'taking the temperature' of investor behavior when it reached extremes of euphoria or despair. This highlights the rarity of true, high-probability moments to make major portfolio shifts.

Investor Peter Lynch's advice highlights that trying to anticipate downturns often leads to missed gains, which can be more costly than the losses from the downturns themselves. The best strategy is often to stay invested rather than waiting on the sidelines for a crash that is impossible to predict.

Widespread public debate about whether a market is in a bubble is evidence that it is not. A true financial bubble requires capitulation, where nearly everyone believes the high valuations are justified and the skepticism disappears. As long as there are many vocal doubters, the market has not reached the euphoric peak that precedes a crash.

A market enters a bubble when its price, in real terms, exceeds its long-term trend by two standard deviations. Historically, this signals a period of further gains, but these "in-bubble" profits are almost always given back in the subsequent crash, making it a predictable trap.

During the dot-com era, savvy investors recognized they were in a bubble but termed it an "iron bubble," believing it would persist. Bailing out too early was a greater risk than riding it to the end, as it meant missing out on significant late-stage gains. This mindset is relevant for navigating today's AI boom.

Contrary to intuition, widespread fear and discussion of a market bubble often precede a final, insane surge upward. The real crash tends to happen later, when the consensus shifts to believing in a 'new economic model.' This highlights a key psychological dynamic of market cycles where peak anxiety doesn't signal an immediate top.

Howard Marks highlights a critical paradox for investors and forecasters: a correct prediction that materializes too late is functionally the same as an incorrect one. This implies that timing is as crucial as the thesis itself, requiring a willingness to look wrong in the short term.